Telegraph trunk circuit



Feb. il, 1947. c. A. DAHLBOM TELEGRAPH TRUNK CIRCUIT FiIed Dec. 22, 1943 3 Sheets-Sheet l /NVE/VTOR CA. DAHLBM B14 /Qw Feb. ii, w47. c. A. DAHLBOM TELEGRAPH TRUNK CIRCUIT Filed Dec.4 22, 1943 5 Sheets-Sheet 2 .NOUN

/A/.VENTOR CADA/#MM BV E. QQ A A r TOR/wf Feb. il, i947. C, A, DAHLBQM ZASAZ TELEGRAPH TRUNK C IRCUIT Filed Deo. 22, 1945 3 Sheets-Sheet 3 Patented Feb. 11, 1947 TELEGRAPH TRUNK CIRCUIT Carl A. Dahlbom, Brooklyn, N. Y., assigner to Bell Telephone Laboratories,

Incorporated,

New York, N. Y., a. corporation of New York Application December 22, 1943, Serial No. 515,255

27 Claims.

This invention relates to teletypewriter line circuits and more particularly to arrangements for terminating one end of such circuits in or at two diierent switchboards in different ollices, for example, in the same city; such two terminations are functionally equivalent to and operate as one terminal in cooperation with the opposite end. In the general case the opposite end may have a multiple terminal of the type of the present invention or a terminal of some more usual type of the prior art.

A specific and exemplary embodiment disclosed comprises a trunk between central oices commonly known as a toll trunk which is of the ringdown type and arranged so that it may be terminated at either or both ends in two different switchboards in two different oices in the same city, each of which provides access to the same end of the intertoll trunk extending to a distant city. The two switchboards in the same city will ordinarily be located in different buildings but may be located in the same building. They may be separated by a considerable distance. Calls may be answered or originated at either one of the switchboards in the same city. The exemplary circuit includes the supervisary features required for use with toll line circuits used as ring-down intertoll trunk circuits and provides for the termination of the trunk at each of two different types of switchboards in the same city.

An object of the invention is the improvement of teletypewriter switching systems.

' A more particular object is the improvement of intertoll telegraph trunk circuits.

Another object is the arrangement of a telegraph trunk circuit so that it includesv supervisory features to provide for its termination in two differentoces in the same city.

A feature of the invention is means for answering an incoming call at either one of two teletypewriter switchboards in two different offices in the same city.

A further feature is a calling-in signal in the form of a break signal of two to four seconds duration which controls means for operatingcall and busy lamps at each of two switchboards in the same city.

A further feature is means in the trunk terminations for extinguishing the call lamps at each switchboard when the call is answered at one of the swtchboards.

Afurther feature is means for impressing a busycondition on the jack of the trunk in one of the switchboards'in the same city when an incoming call is answered at the other switchboard.

A further feature is means at either one of the switchboards at which an incoming call has been answered for momentarily grounding a supervisory lead in the trunk termination to control a supervisory signal in the cooperating cord circuit in response to a recall signal from the distant end of the line in the form of a seven-second break signal.

. A further feature is means in the trunk termination at the switchboard where the call has been answered for applying a permanent ground to the supervisory lead extending to a connecting cord circuit to control a supervisory signal to indicate a disconnect signal in response to a ten-second break ap-plied at the distant end of the trunk.

The manner of accomplishing the above and other objects and the manner of accomplishing the above and other features will become apparent from the following description when read with reference to the associated drawings in which:

Fig. 1 shows a distantl teletypewriter subscribers station connected to a distant teletypewriter central ofce which is in turn connected to a terminating repeater which is located at the same oflice in which the trunk termination per Fig. 2

appears;

Fig. 2 shows the telegraph trunk termination at one of the switchboards in one of the offices in a particular city;

Fig. 3 shows a telegraph trunk termination in a second switchboard in a second oflice located in the same city as that in which the facilities per Fig, 2 are located;

Fig. 4 is a diagram indicating the manner in which Figs. 1, 2 and 3 are arranged each in ree lation to the other to form a complete system with terminations of a trunk in two offices in the same city; and

Fig. 5 is a diagram indicating the manner in which Figs. 2 and 3 and the terminating repeater per Fig. 1 are arranged each in relation to the other to form a complete system with terminations in two offices at veach end of the channel.

Refer now to Figs. 1, 2 and 3. At the left of Fig. l is a rectangle ll which represents a distant teletypewriter subscribers station. It may be any one of a, large number of such stations each 0f which is connected by an individual telegraph channel such as 102 to a teletypewriter switching ofice in the distant city represented by rectangle lll3. From the distant teletypewriter `ofnce |03 an intertoll trunk channel |04 extends to the trunk terminating repeater |65. From the terminating repeater which is located in the same city as that in which the facilities per Fig. 2 and Fig. 3 are assumed to be located, a telegraph channel |06 extends to a trunk termination switch |01. From the trunk termination switch |01 two branches |08 and |06 under control of switch |01 extend to telegraph trunk terminating equipment H6 which is located in one switchboard and which is located in another switchboard in the same city. In the idle condition the transmission paths of the two ring-down intertoll trunk circuits are held vin a marking condition. A circuit may be traced from battery ||2 in Fig. 1 through resistance H3, contact of relay ||5 to the apex of relay |i6. From the apex of relay I6 one branch extends through the top Winding or relay i6, resistance |i and channel |64 to the distant teletypewriter oiice |03 where it terminates in negative battery. From the apex of relay ||6 the second branch extends through the lower Winding of relay H6 and resistance |18 to ground. The effect of the current ilowing through the top winding of relay I6 which tends to operate the armature cf relay ||'6 toward the left to engage contact IE6 is dominant over the eiect of current nowing through the bottom or biasing winding of relay ||6 which is tending to operate the armature of relay ||6 toward the right to engage contact |20. The armature yof relay H6, therefore, engages its marking contact ||9. A circuit may then be traced from negative battery |2| through resistance |22 and contact ||9 to the apex of relay ||5. From the apex of relay ||5 one branch of the circuitextends through the bottcmwinding of relay ||5 and resistance |23 to ground to provide biasing current for relay i5. From the apex of relay ||5 a second branch extends through the top winding of relay H5, resistance |24 and channel |66 to point |25 where parallel branches are formed. One branch extends through conductor |26 and contact |21 of relay |28 to another parallel branch at point |20. One of the parallel branches extending from point |29 leads through contact |42 of relay |3|, Contact |43 of relay |28, resistance |64, conductor |08, top winding of relay |36, contact |31 of relay |38, contact |39 of relay |40 and resistance |4| to positive battery. From point |29 the second branch extendsthroueh contact |30 of relay |3|, contact |32 of relay |3|, contact |34 of relay |26, resistn ance |35, conductor |09, resistance |45, winding of relay |46, contact |231 of relay |68 andresistance |40 to positive battery.

Attention is called to the fact that both or" the transmission branches |06 and |00 at this time Vare terminated in. positive battery connectedto resistances |4| and |40, respectively. The current flowing through the top winding of relay i l5 which is tending to actuate the armature of relay ||5 to engage its marking contact is dominant over the effect of current flowing 'through the bottom winding of relay i5 which is tending to actuate the armature of relay to engage its spacing contact |50 and the armature of relay i5 is, therefore, maintained in engagement Withits marking contact Hd. It Vwas pointed out in tracing the circuit above that parallel branches tor |26. The path through conductor |26, therefore, forms a shunt around resistance |5| while the relays |28 and |3| are in the condition shown in Fig. 2. A circuit may be traced from battery through resistance |52 and the bottom winding of relay |36 to ground. This supplies biasing current to the bottom winding of relay |36 which tends to actuate the armature of relay |316 towards the left to engage its contact |53. The effect of current owing through the top winding of relay |36 over the path traced above tends to actuate the armature of relay |36 toward the right to engage its marking contact |54 and the eiect of this current is dominant over the effect of the biasing current so that the armature of relay |36 engages contact |54. The winding of relay |06 in Fig. 3 is energized by the eiiect of current flowing through conductor |06 in the path traced above so that contacts |55 and |56 are closed.

When contact |54 of relay |36 in Fig;V 2 is closed, .a circuit may be traced from ground through contact |54 and the winding of relay |52 to battery actuating the armature of relay |5 to maintain contact |58 closed while contact |50 is open. When contact |56 .of relay |46 in Fig. 3 is closed,7a circuit Vmay be traced from ground through Contact |56 and the winding of relay |60 to battery energizing relay |60 which closes contact |6| and opens contact |62.

The above is the condition of the circuits per Figs. 1, 2 and 3 at the start of operations, Other relays in Figs. l, 2 and 3 at such time are in the released condition as shown on the drawings.

Incoming call When a call is incoming from the distant teletypewriter cnice |03 a spacing signal of from two to four seconds duration is impressed on channel |04 extending from the distant oiice |03 to the terminating repeater |05, In response to this the eiect of the current in the top winding of relay ||6 is reduced so that the effect on the current in the bottom winding of relay 6 be-v comes dominant and the armature of relay H6 is actuated to yengage its spacing contact |20. As a result of this positive battery is connected through resistance |63 and contact |20. Since positive battery.is connected also through resistance |4| and resistance |40 at the opposite ends of this circuit, no current will flow through the top winding of relay |36 or through the winding of relay |46. The armature of. relay |36 will be actuated to closeitsspacing contact |53 and the contacts of relay |46 will be opened. The description of the sequenceof eventsin Fig. 2 in response to the spacing Signal for Ythe incoming call will rst be described. y

The opening of contact |54 releases relay |51. Contact |59 is closed. This establishes a circuit from battery through contact |56, winding 'of relay |64 and contact 255 of relay 206 to brush 251 of interrupter 256 which closes a circuit to ground sixty times per minute'. Relay |64 will be operated and released, closing and opening contact |65 once per second while the spacing signal from oce |03 persists. When-contact |65 is closed a circuit may be traced from ground, through contact |65 and the winding of rotary stepping magnet |66 of the two-bank selector switch |67 to battery, energizing and deenergizing rotary stepping magnet |66 once per second. The rotatable arms |60 and |69 associated with banks 1 and 2 of selector |61, respectively, are inthe positions indicated when in the tions,

released condition. Each time rotary steppng magnet |66 is energized, rotatable arms |68 and |69 are stepped in unison in a counter-clockwise direction to engage successive terminals on their respective banks. The oli-normal spring assembly is actuated to close contacts |1| and |12. Contacts |1| and |12 remain closed while arms |68 and |69 are in any other than their normal released positions. Arms |68 and |69 will be stepped from one to four steps, dependent upon the duration of the spacing signal.

When arm |68 engages its second, third or fourth terminals a circuit may be traced from ground, through contact |53, arm |68, terminals 2, 3 or 4 of bank 1 of selector 61, winding of relay |13 and contact |19 to battery, operating relay |13 and closing contact |14. Upon completion of the spacing signal, contact I9 of relay ||6 will be reclosed. The top winding of relay |36 will be reenergized, closing Contact |54 and reoperating relay |51. This, in turn,- recloses contact |58, A circuit may then be traced from battery through contact |58, winding of release magnet 290 of selector |61 and contact |1|, of off-normal spring assembly |10, to ground, energizing release magnet 290 which immediately restores arms |68 and |69 to their original posi- The circuit through the winding of relay |13 is thus opened and relay |13 releases, opening contact |14. Relay |13 is a slow-to-release relay so that contact |14 remains closed once relay 13 is operated for an interval long enough to permit relay |15 to operate over a circuit which may be traced from battery, through contact |58, contact |14, contact |16 of relay |15. winding of relay |15 and contact |11 of relay |18 to ground. When relay |15 operates it locks over a circuit which extends from battery, through contact |19 of relay |40, contact |80 of relay |15, winding of relay |15 and contact |11 to ground.

During the interval while the trunk terminating circuit per Fig. 2 is idle, the idle indicator lamp |8| associated with jack |82 of the trunk is lighted. It is pointed out that the trunk termination per Fig. 2 has a plurality of terminations in the switchboard in which the trunk per Fig. 2 is terminated. The tip, ring and sleeve connections |84, |85 and |86 of jack |82 are connected in parallel to jacks at a number of positions of the switchboard, Similarly, the answering lamp conductor |81 is connected in parallel through the filament of an answering lamp, such as lamp |83, to ground at each of the positions where jack |82 appears. Similarly the idle indicator lamp conductor |88 is connected through a lamp filament of a lamp, such as lamp |8|, to ground at each of the appearances of jack |82. Whenever the switchboard in which iacks |82 are located is being operated, key |89 of the idle indicator relay chain circuit |90 is operated to close contact |9|. This establishes a circuit from ground through contact |9| and the winding of relay |92 to battery, energizing relay |92 and closing all of its associated contacts. Relay |92 controls a number of relay chain circuits, such as the one shown in Fig. 2.

While the trunk termination per Fig. 21s idle, a circuit may be traced from ground, through the filament of idle indicator lamp |8|, contact |93 of relay |94 and contact 295 of relay |92 to battery, lighting idle indicator lamp |8|.` Since the trunk termination per Fig. 2 is now engaged to receive an incoming call lamp |8| is extinguished. This is performed by the closing of contact |12 ofthe oft-normal spring assembly |10.

When contact |12 is closed, a circuit may be traced from battery, through contact 12 and the winding of relay |94 to ground, operating relay |94 which extinguishes lamp |8| by opening contact |93. Relay |15 is operated before contact |12 is reopened. This maintains relay |94 operated over a circuit which may be traced from battery, through contact |95 of relay |15 and the winding of relay |94 to ground. The operation of relay |94 also transfers the idle indicator circuit to the next'relay in the chain by closing contacts |96 and |91. The operation of relay |15 also establishes a circuit from battery, through contact |98 and the lament'of answering lamp |83 to ground, lighting answering lamp I 83 as an indication that a call. incoming from jack |82 is Waiting to be'answered.

The operation of relay |15 also establishes a circuit from ground, through contact |99 and the night alarm circuit 200 to battery, not shown, to operate night alarm circuit 200.

The closing of contact |95 of relay |15 also establishes a circuit from battery, through contact |95 and the bottom and top windings of relay |40 in series to ground. Relay |40 is wound differentially, that is to say, when current flows through both windings of relay |40 in series as in the present instance, the elfects of the current in the two windings oppose each other so that relay |40 remains in the released condition as shown. Attention is called, however, to the fact that the sleeve conductor |86 of jack |82 is connected to the junction of the top and bottom windings of relay |40. As a result of this, when negative battery is connected through contact |95 the negative potential between the junction of the two windings of relay |40 and ground is impressed on the sleeve of jack |82. This serves as a busy indication at the multiple appearances of jack |82.

The description of the operation of Fig. 3 will now be resumed. The opening of contact |56 deenergizes the winding of relay |60, opening contact |6| and closing contact |62. The closing of Contact |62 establishes a circuit from battery, through contact |62, resistance 20|, winding of rotary stepping magnet 202 of two-bank selector 203, contact 204 of relay 205, through interrupter 206 to ground. Interrupter 206 is arranged to break the circuit to ground at the rate of sixty times per second. The rotary stepping magnet 202 will be energized and deenergized once per second. Brush arms 206 and 201 are normally in the position-s indicated. Each time rotary magnet 202 is energized rotatable brush arms 206 and 201 are stepped in unison, one step, in a counterclockwise direction. When arm 206 engages the second terminal of its bank a circuit may be traced vfrom ground, through arm 206, second terminal of bank 208, winding of relay 209 and contact |62 to battery. operating relay 209. In response to the reclosure of the line from the distant station at the termination of the breaking interval within the interval of two to five seconds relay |46 will be reoperated. This, in turn,- reoperates relay |60. When the selector brush arms 206 and 201 are stepped oil-normal the oilnormal spring assembly 2|0 of selector 203 is actuated to close contact 2|| and open contact 2|2. A circuit 4may then be traced from ground, through contact |55, contact 2||, contact 2|4. release magnet 2|5 of selector 203, resistance 2|6 and contact |6| to battery. This restores arms 206 and 201 tonormal. y

'Relay 20,9 is a slow-to-release relay. During the interval after relay |46 operates and before relay 209 releases, a circuit may be traced from ground through contact |55, contact 2| l, contact 2|3, Switch arm 281, contact 2|1 of relay 218, winding of relay 219 and contact 220 ci relay |48 to battery operating relay 2|9. When relay 2|9 operates, it locks over a circuit from battery through contact 220, winding of relay 2|9, contact 2|1 of relay 2&8 and Contact 22| of relay 2|9 to ground. Relay 2 |79 will remain locked until the operator at the switchboard where Fig. 3 is terminated responds to the call. When relay 2| 9 operates, a circuit may be traced from battery through contact 222 of relay 2|9 and lamentof lamp 223 as well as through the filaments of other lamps in parallel with lamp 223 at all switchboard positions to ground. The operation of relay 2|9 also establishes a circuit from battery through contact 224 and the winding of relay 225 to ground operating relay 225. A circuit may then be traced from ground through a source of alternating current 228, contact 221 of relay 225 and the lament of lamp 228, as Well as the filaments of other lamps connected in parallel with the filament of lamp 228 at other positions in the switchboard, to ground. It is pointed out that one relai7 such as225 and one source of alternating current such as 226 are connected so as to serve a plurality of trunks. The left-hand terminal of relay 225 is connected in parallel to a contact such as 224 in each of a number of diierent trunks. The right-hand terminal of contact 221 is connected in parallel to a number of conductors which extend to groups of lamps such as lamp 228. Relay 225 and alternating current source 225 constitute an alternating current supply circuit for busy lamps such as 228 in the oil'ice where Fig. 3 is located. The object of the use of this busy lamp circuit is to reduce the drain on the direct current batteries in the oilice.

The operation of relay 2| 9 also establishes a circuit from ground through contact 229 of relay 2 I9 through the night alarm circuit-.indicated by rectangle 239. If the night alarm circuit is closed a signal will be operated to attract the operators attention. The night alarm circuit 230 is Connected in parallel to the right-hand terminal of a number of contacts such as 229 so that the single night alarm circuit 230 will serve a large number of trunk terminations such as Fig. 3 in a well understood manner.

The call lamps and busy indications are now displayed at both switchboards. When the operator at either switchboard answers the call, the jack appearance at the unanswered switchboard is made to test busy for the duration of the call.

The circuits which cooperate with the trunk circuit herein at the two central oi'ces located inthe vcity where Figs. 2 and 3 are terminated are disclosed in kdetail in Patent 2,220,931, J. A. Krecek, November 12, 1940, and in allowed application Serial No. 424,436, C. A. Dahlbom et al.. led December 26, 1941, respectively. The cord circuit per Fig. 2 in Patent 2,220,931 may be connected to Fig. 2 herein to extend the connection through the system disclosed in Patent 2,220,931. The cord circuit per Fig. 2 of application Serial No. 424,436 may be connected to Fig. 3 herein .to extend the connection through the system dis.- closed in the application Serial No. 424,436. Patent 2,220,931 and application Serial No. 424,436 are incorporated herein. by rei rence as though fully set forth herein. f

Answering an incoming call through Fig'. -2

The manner in which the trunk termination per Fig. 2 herein responds when the answering end of the cord circuit disclosed in Patent 2,220,- 931 is connected to jack |82 will now be described. In this description the conditions imposed on jack |82 by the cooperating cord for the different operations will be described. The manner in which the cord circuit in Patent 2,220,931, responds may be .understood from reference to Patent 2,220,931. Y

When the answering end of the cord circuit is inserted in jack |82, negative battery will be im pressed `on the sleeve connection |85 of jack |82 of the same-magnitude as the negative battery connected through rcontact of relay |15. A circuit may be traced from the sleeve of jack |82 through conductor |86 to the junction of the windings of relay |40. One path extends from the junction through the bottom winding of relay l|40 and contact |95 to negative battery. The lower winding of relay |48 will be deenergized. A second path may be traced from the junction of relay |40 through the top winding of relay |40 .to ground. Relay |40 will be energized to close its make contacts. When relay |40 is operated, the transmission path incoming from the distant o'ice |03 in Fig. 1 which formerly extended through contact |39 and resistance |4| to positive battery will be opened at contact |39 and extended through contact 23|, resistance 232 andthe ring connection |85 of jack |82 through the ring of the connecting cord circuit. The closing of contact 233 of relay |40 establishes a circuit from battery through Contact 233 and the winding of relay |94 to ground to maintain the idle indication. The opening of contact |19 of relay |40 releases relay |15. The closing of contact 234 of relay |40 establishes a circuit from ground through resistance 235, contact 234 and the bottom winding of relay 236 to battery operating relay 236. The closing of contact 231 of relay 236 establishes a circuit from' ground through the contact 231 and conductor'Z 38 which extends between the offices where the trunk -terminations per Fig. 2 and Fig. 3 are located, through the winding of relay 2 I 8 to battery operating relay 2|8 in Fig. 3. The closing of contact 239 establishes a circuit from ground through contact 239, contact 248 of relay |28, winding of relay |3| and resistance 24| to battery operating relay |3|. The closing of contact 242 of relay 239 will prepare a holding path extending from battery through the top winding of relay 236 and contact 242 to contact 243 of relay. |38, which is open, foruse at a later time. The release 'of relay |15 extinguishes the answer lamps such as |83 by opening contact |98. The open-i ing of contact |99 of relay |15 disconnects the night alarm circuit. The operation of .relay |3| by closing contact |32 applies a holding termination to the unanswered trunk 4termination per Fig. 3. The operation of relay |3| also removes the short-circuit which extended through` conductor |26 around resistance |5| by opening contacts |38' and |42. Removal of this short-circuit provides the correct loop current. The opera-v tion of relay I3! by openingV contact 245 opens the operating path of relay' |28. This removes the danger of transmission circuit interruption should the operator at the switchboard where the 4trunk terminal per Fig. 3 is located accidentally connect'a cord to the jack in Fig. 3. This will be Vunderstood ,from .the description below.

9.. The operation of relay 2|8 in Fig. 3 by opening contact 2|1 will release relay 2|9. The release of relay 2|9 will in turn extinguish the lamps such as 223 by opening contact 222. The opening of contact 229 of relay 2|9 will disconnect the night alarm circuit 236. The closing of contact 246 of relay 2|8 will establish a circuit from negative battery through contact 246, resistance 241, conductor 248 to the sleeve of jack 249 and land to the sleeve of other jacks in parallel with jack 249 at all positions in the switchboard where Fig. 3 is terminated. The closing of Contact 259 of relay 2|8 will establish a circuit from battery through contact 25|) and the winding of relay 225 to ground to maintain all of the busy lamps If the plug or" the cord circuit is inserted in jack 249, battery on the sleeve oi the plug will be connected through the winding of relay |48 to ground operating relay |48. The closing of contact 260 of relay |48 will connect the transmission path n Fig. 3 to the transmission path of the connected cord circuit. The opening of contact 226 of relay .|48 will release relay 2|9. The opening of contact 222 of relay 2|9 will extinguish all of the answering lamps such as lamp 223. The opening of contact 229 of relay 2|9 will release night alarm circuit 235. The closing of contact 26| of relay |48 will establish a circuit frorn battery through contact 26| and the winding of relay 262 to ground operating relay 262. The operation oi relay 262 will establish a circuit from negative battery through contact 263 and the winding of relay 225 to ground to maintain the busy lamp such as lamp 228 lighted. The operation of relay 262 will establish a circuit from battery through contact 264, conductor 265 into Fig. 2 through the winding of relay |18 to ground operating relay |18. The operation of relay |18 will establish a circuit from ground through contact 266, contact 245 of relay |3I, winding of relay |28, and resistance 261 to battery operating relay |28. The operation of relay |18 will establish a circuit from negative battery through contact 268 and the top and bottom windings of differential relay |46 in series so that negative potential is iinpressed on the sleeve of jack |82 over conductor |86 from the junction of the windings of relay |45. This will provide a busy indication on all jacks such as jack |82 in the switchboard where Fig. 2 is terminated. The operation of relay |18 by opening contact |11 will release relay |15. This in turn will extinguish the lamps such as lamp |83 by opening contact |98. The opening of contact |99 of relay |15 will disconnect the night alarm 260. The operation oi relay |28 will apply a holding termination to the unanswered trunk termination per Fig. 2 by connecting battery through resistance 269, contact 216 and resistance |44 to conductor |08. The operation of relay |28 by opening contact |21 will remove the shunt around resistance |5| and by closing contact 21| will connect resistance |5| in the transmission path to the transmission channel through Fig. 2. This provides the proper loop current. The operation of relay |28 by opening contact 24|) will break the path extending through the winding of relay |3| so that relay I3! cannot be operated accidentally should a cord circuit plug be inserted in jack |82. The operation of relay |18 by closing contact 268 operl@ ates relay |94 to maintain the idle indicator circuit in operation.

Receiving recall signals 'The manner in which recall signals are transmitted through the trunk terminations per Fig. 2 and Fig. 3 will new be described. First, the manner in which they are transmitted through Fig. 2 will be described. A recall signal is received by means of a spacing signal of approximately seven seconds in duration. In response to the spacing signal as has been shown the armature of relay |36 will be operated to engage a spacing contact |53. This in turn releases relay |51 in the manner described. In response to this the selector arms |68 and |69 will be operated in the manner described to approximately the seventh terminal on each bank. When brush arm |68 engages the sixth terminal of its bank a circuit may be traced from ground through contact |53, brush arm |68, sixth terminal of the selector bank associated with brush arm |68 and the winding of relay 212 to battery operating relay 212. A circuit may then be traced from the tip conductor of jack |82 through conductor |84, contact 213 of relay 236, contact 214 of relay 215, contact 216 and contact |54 of relay |36 to ground. The path through contact |54 to ground is available as soon as the recall spacing interval is ended. When the armature of relay |36 engages contact |54, relay |51 is reoperated. Y This restores selector |51 to its normal position. Relay 212 is a slow to release relay. Ground through contact |54 is connected momentarily prior to the release of relay 212 to the tip conductor |54 of jack |32 and through the tip of the connected cord circuit to actuate a flashing recall signal in the connected cord. When relay 212 releases battery is connected through contact 211 to the tip conductor. The flashing recall signal transmitted from Fig. 2 therefore consists in replacing battery normally connected to the tip conductor of jack |82 with ground momentarily and then reconnecting battery to the tip conductor.

The manner in which a recall signal is transmitted through Fig. 3 will now be described.V

When a recall signal in the form of a spacing condition of approximately seven seconds duration is received by the trunk termination of Fig. 3 brush arms 206 and 201 of selector 283 will be advanced to their sixth, seventh or eighth contact. Relay 269 is operated in response to this in a manner which has been described. When relay |46 reoperates at the end of the spacing period relay |69 will be reoperated as described heretofore t0 deenergize the rotary magnet and relay 299. Relay 299 is a slow to release relay. During the releasing time of relay 209 ground is connected to a circuit which extends through contact |55, contact 2|| of the oli-normal assembly 2 |0,.contact 2|3 of relay 269, selector arm 291, sixth terminal of bank 209 and contact 218 of relay 265 to the ring conductor 28|) of jack 249 and through the ring of the connected cord circuit plug into the cord circuit. When relay 209 nally releases ground will be disconnected from the ring. This momentary grounding of the ring lead will cause the connected cord circuit to register a flashing recall signal. The flashing recall signal will be retained until the operator responds and will be. retired by the action of the operator as a function of the cord circuit entirely independent of any further operation of the intertoll vtrunk circuit.`

The manner in which disconnect signals are received through the trunk termination per Fig. 2 and Fig. 3 Will now be described. First, the manner in which the disconnect signal is received through Fig. 2 will be described.

Adisconnect signal transmitted from the distant operatoi or station opens the line circuit for approximately ten seconds to operate the armature of relay |36 to engage contact |53. This releases relay |51 in a manner heretofore described. This in turn operates relay |64 in a manner which has also been described. Selector brush arms |68 and |68 will be stepped until they engage the tenth terminals on their respective banks. `When brush arm |68 engages its ninth terminal a circuit may be traced from ground through contact |53 of relay |36, brush arm |68, ninth terminal on the corresponding selector bank, and the bottom Winding of relay 215 to battery operating relay 215. The operation of relay 215 connects ground through the upper or holding Winding of `relay 215, contact 216, contact* 213 of relay 236 and conductor |84 to the tip of jack |82 and through the tip of the plug of the connected cord circuit to operate the disconnect signal in the cord. In this case the reopening of the line circuit at the distant end will not change the disconnect signal into a flashing-r recall signal.

The closing of the line at the completion of the tenvsecond open reoperates the armature of relay |36 to close its marking Contact |54. Relay |51 reoperates and restores selector |61 to normal in the manner which has been described. When the selector reaches step 10 the conductor from the interrupter is opened.

The manner in which a disconnect signal is transmitted through the trunk termination `per Fig. 3 will now be described.

When a disconnect signal in the form of a spacing condition of approximately ten seconds duration is received over conductor |09, relay |46 is released. Relay 209 will be operated during the interval While the selector brush arm 206 engages with its fifth, sixth, seventh and eighth.

contacts but will be released when brush arm 206 is advanced to its ninth contact. When brush arfr'n 206 engages its ninth contact a circuit may be `traced Vfrom ground through brush arm 206 and the top winding of relay 205 to battery operating relay 205. The operation of relay 205 will prepare a locking path for this Vrelay over a circuit which may be traced from battery through contact 263 of relay 262, bottom winding of relay 205, contacts 28|, 282 and 283 of relay 205 to open contact 2| 2 of the off-normal spring assembly 2|0. Contact 2|2 will close When selector 203 returns to normal. The operation of relay 205 by opening contact 204 will disconnect ground through interrupter 206 from rotary magnet 202 of selector 203. The object of opening contact 204 yis to prevent excessive Wear of the rotary selector in the event that a trouble condition causes a spacing condition of longer than nine seconds. When the disconnect spacing period is ended and selector 203 is released, contact 2|2 will be reclosed, locking relay 205 operated through its bottom winding. The reoperation of relay |46 at the end of the disconnect interval establishes a circuit from ground through contact |56 of relay |46, contact 282 of relay 205 to ring conductor v280 of jack 249 and throughI the ring of the plug' ci the connected cord circuit to 12 light'the cord circuit 4supervisory lamp steadily as a disconnectv signal. Whenvrelay |46 is operated at the end of the disccnnectsignal relay |60 will also reoperate. This in turn restores selector 203 to normal.l Since' ground is suppliedthrough contact 2 l2 of the ofi-normal spring assembly 2 0 through the contacts of relay 205 to the ring conductor of the jack, subsequent release of relay y Sending a recall signal from the switchboard through Fig. 2 toward the distant oice The manner in which a recall signal is transmitted from the switchboard at which Fig. 2 is located Will now be described. In order to transmit a recall signal through Fig. 2 a key in the cord circuit is operated to apply positive battery to tip conductor |84 oi jack |82 and a circuit may then be traced through contact 213 of relay 236, contact 214 of relay 215, contact 211 of relay 212 and the top Winding of relay |38 to negative battery operating relay 38. When relay |38 operates it locks over a circuit which may be traced from ground through contact 243, bottom winding of relay |38, contact 205 of off-normal spring assembly |10 to negative battery.

When relay |38 is operated contact |31 is opened which opens the transmitting path to the distant office. Contact 286 is closed. This connects negative battery through resistance 201, contact 286, contact 23| of relay |40 and resistance 232 to transmitting ring conductor |85. This holds the cord circuit repeater in the marking condition during the open circuit period of the recall signal. The opening of the circuit through the top conductor of relay |36 permits Contact |53 to close. This in turn releases relay |51. The release of relay |51 operates relay |64. This in turn steps the selector. These operations have been described heretofore. Contact 285 of the ofi-normal spring assembly is open but since a parallel path to battery is closed through brush arm |69 of selector |61 to battery relay |38 is maintained locked in operated position. This locking condition is maintained until brush arm |63 engages terminal No. 7. The paths extending from terminal No. 7 are open at this time. Relay |38 is therefore released to close the transmitting channel through Fig. 2. Relay |36 thereupon closes contact |54. This results in the transmission oi a seven second open line signal toward the distant oice |03 as a flashing recall signal. When brush arm |68 engages its sixth terminal relay 212 is operated over a circuit heretofore traced. This connects ground through contacts |54, 216, 214 and 213 to tip conductor |84 of jack |82 and through the tip of the plug of the connected cord circuit to actuate the ashing recall signal in the home cord after relay |36 engages contact |54.

Sending adisconnect signal from the switchboard through Fig. 2 to the distant ofce The manner in which a disconnect signal is transmitted from the switchboard through Fig. 2 to the distant office will now be described.

A disconnect signal is transmitted by operating a key in the cord circuit to connect positive battery to tip conductor |84 of jack |82. This operates relay |38 in a manner which has been described. Relay |38 in turn opens the transmission circuit toward the distant office and locks through contact 285 over a circuit which has been traced. The lock is maintained when selector |61 is stepped in response to the open signal, in a manner which has been described, from negative battery connected to terminals of the selector bank associated with brush |69. The operator removes the cord from jack |82. This releases relay |40. A circuit may then be traced from negative battery through contact |19 of relay |40, terminal No. 7 of bank 2 of selector |61, brush arm |69, bottom winding of relay |38, and contact 243 to ground to maintain relay |38 operated. This maintains relay 236 operated over a circuit from ground through contact 243, contact 242 and the top winding of relay 236 to battery. This in turn maintains the busy condition on the sleeves of jacks such as 249 in the switchboard Where Fig. 3 is located until the disconnect signal is completed.

Sending recall and disconnect signals from the switchboard through Fig. 3 toward the distant office In transmitting recall and disconnect signals from the switchboard in which Fig. 3 is terminated toward the distant ofce in Fig. l the transmission circuit extending through Fig. 3 is not opened in the terminating trunk as was the case for the trunk termination per Fig. 2. Recall and disconnect signals are transmitted from the operators position circuit connected to the cord circuit which cooperates with Fig. 3 and does not involve the operation of apparatus in Fig. 3. These operations Will therefore not be described.

Outgoing calls In the case of a call originating either in the switchboard to which Fig. 2 is connected or to which Fig. 3 is connected the calling cord of the cord circuit is connected to jack |82 or to jack 249. In either case the calling signal is transmitted by the operation of a key in the cooperating cord to send an open signal of from two to ve seconds. The answering lamps in the corresponding trunk termination will not be lighted as relay |15 in Fig. 2 and relay 2|9 in Fig. 3 are not operated on calls originating in the switchboards where they are located.

Out-of-service feature When the termination circuit disclosed in Fig. 2 is out of service due to the opening of the path through the line winding of relay |36, as a result of a trouble condition, further operation of selector |61 is avoided, once the selector arm |68 reaches terminal I0, by the operation of relay 296. Relay 296 operates in series with relay 215 on step 10 of selector bank 1 of selector |61 over a circuit from ground through contact |53, arm |68, terminal l and the windings of relays 296 and 215 to battery. This disconnects interrupter 258 by opening contact 255. The arms |68 and |69 of selector |61 will thereafter remain on the tenth terminal of their respective banks until the trouble is cleared.

An. out-of-service feature is included also in the termination circuit disclosed in Fig. 3. In Fig. 3 the interrupter is disconnected when ground in connected through brush 206 and contact I0 of bank through the top winding of relay 205 to battery opening contact 204 after an open `'signal of ten seconds or more in duration.

Termination in two offices at each end of channel As mentioned in the second paragraph on page 1 the double-ended trunk of the present invention may be located in two different switchboards in two different cities interconnected by a single channel. The gures are then arranged, as would be obvious to one skilled in the art, as shown in the diagram per Fig. 5 and the operation of the trunk circuit at each end is the same as described in the foregoing.

What is claimed is:

1. In a telegraph switching system, a telegraph trunk circuit, a rst telegraph central ofce connected to one end of said trunk, a second and a, third telegraph central oilice connected in parallel to the other end of said trunk, means in said trunk for transmitting -a trunk callingsignal from said rst office to both said second and said third ofces simultaneously, means in said trunk -for transmitting communication signals from said first office to either one of said second and third ofces, and means in Said trunk circuit for transmitting supervisory signals from said first ofce to either one of said second and third o'ices.

2. In a telegraph switching system, a telegraph trunk circuit, a rst telegraph central oice connected to one end of said trunk, a second and a third telegraph central oce connected in parallel to the other end of said trunk, means in said trunk for transmitting a trunk calling signalfrom said second or said third oice to said iirst olice, means in said trunk for transmitting communication sign-als from said second or said third oli'ice to said rst oftlce, and means in said trunk for transmitting supervisory signals from said second or said third oiiice to said rst oce.

3. In a telegraph switching system, a telegraph channel, a telegraph repeater in said channel, a rst and a second telegraph central oice connected to one end of said channel, means in each of said central offices for transmitting a channel calling signal over said channel, means in each of said central oiiices for transmitting communication signals over said channelf and means in each of said central offices for transmitting supervisory signals over said channel.

4. In a telegraph switching system, a telegraph channel, a telegraph repeater in said channel, a rst telegraph central office connected -to one 'end of said channel, a second and a third telegraph central office connected in parallel to the other end of said channel, means in said channel for transmitting a calling signal over said channel from said rst oilice to said second and third offices simultaneously, means in said channel for transmitting"communication signals over said channel to said second office or to said third oillce, and means in said channel for 4transmitting supervisory signals over said channel from said first office to said second or said third ofllce.

5. In a telegraph switching system, a rst manual central telegraph subscribers switchboard, a second and a third manual central telegraph subscribers switchboard, a, telegraph channel connecting said first switchboard to said second and third sWitchboards in parallel, a telegraph repeater in said channel, means in said system for transmittingcommunication signals from said first switchboard to said second or said third switchboard, Vand means in said system for transmitting a calling signal from said rst 15 switchboard to said second or said third switchboard.

6. In a telegraph switching System, a iirst manual central telegraph switching oice, a second and a, third manual central telegraph switching office, a telegraph channel Connecting said first oiiice to said second and third offices in parallel, a telegraph repeater in said channel, means in said system for transmitting communication signals from said rst ofce to said second or said third ofce, and means in said system for transmitting a recall signal from said iirst oflice to said second or said third oiiice.

'7. In a, telegraph switching system, a rst telegraph ofdce, a second and a third telegraph oflice, a telegraph channel connecting said first cnice tor said second and third offices in parallel, a telegraph repeater in said channel, means in said system for transmitting communication signals from said rst office to said second or said third orlice, and means in said system for transmitting a disconnect signal from said first office to said second or said third oice.

8. In a telegraph switching system, a first telegraph central oice, a second and a third telegraph central oice, a telegraph channel connecting said iirst cnice to said second and third oliices in parallel, a telegraph repeater in said channel, means in said system for transmitting communication signals from said first cnice to said second or said third oice, means in said system for transmitting a calling signal from said first oice to said second or said third oiiice, and means in said system for transmitting a disconnect signal from said' first oiice to said second or said third ofce.

9. In a telegraph switching-system, a, iirst telegraph oilice, a second and a third telegraph ofce, aV telegraph channel connecting said rst oice to said secondand third oilices inV parallel, a telegraph repeater in said channel, means in said system for transmitting communication signals from said first oice to said second or said third oice, means in said system for transmitting a calling Signal from said first office to said second or said third oice, and means in said system for tralrismittingv a recall signal from said first oihce to said second or said third oiiice.

l0. In' a telegraph switching system, a first telegraph cnice, a second and a third telegraph office, a telegraph channel connecting said rst oliice to said second and third oflices in parallel, a. telegraph repeater in said channel, means in said system for transmitting communication signals from s'aid iirst ofce to said second or said third ofc'e, means in said system for transmitting a recall signal from said iirst ofce to said second or said third oliice, and means in said system for transmitting a disconnect signalV from said first oice to said second or said third office.

11. In a telegraph switching system, a first telegraph oice, a second and a, third telegraph oiiice, a telegraph channel connecting said iirst oilice to said second and third offices in parallel, a telegraph repeater in said channel, means in said system for transmitting communication signals from said iirst office to said second or said third cnice, means in said system for transmitting a calling signal from saidiirst oiiice Vto said second or said third oiiice, means in said system for transmitting a recall signal from Said Vilrst oiiice to said second or said thirdV oiiice, and means in saidisys'tem for transmitting a disconnect signal from said r'st office to said second or saidthird oliice.

12. lIn a telegraph switching system, a rst telegraph central oihce, a second and a third telegraph central oliice, a telegraph channel corinecting said iirst ofce to said second and third offices in parallel, a telegraph repeater in said channel, means at either said second or said third oiiice Vfor transmitting telegraph communication signals over said channel to said rst oflice,and

means in said system for transmitting supervis-V ory signals from said second or said third olce to said first oiice. A

13. In a telegraph switching system, a first telegraph central oflice, a second and a third telegraph central oce, a telegraph channel connecting said iirst oice to said second and third onices in parallel, a telegraph repeater in said channel, rmeans at either said second or said third omce for transmitting telegraph communication signals over said channel to said 'first oiiice, and means at either said second or said third oiice for transmitting channel calling signals from said second or third office to said first oiiice.

14. In a, telegraph switching system, a irstv telegraph central ofce, a second and a third telegraph central oflice, a telegraph channel connecting said nrst oilice to said second and third ofces in parallel, a telegraph repeater in said channel, means at either said second or said third ofce for transmitting telegraph communication signals over said channel to said first ofrice, and means in said second and said third cnice for transmitting recall signals from said second or said 'third office to said rst ofce.

l5. In a telegraph switching system, a first telegraph central cnice, a second and a third telegraph central oiiice, a telegraph channel connecting said first office to said second and third oices in parallel, a telegraph repeater, in said channel, means at either said Second or said third cnice for transmitting telegraph communication signals over said channel to said first oce, and means Yat said second or third office for transmitting disconnect signals from said second omce to said rst oiiice.

16. In a telegraph switching system, a rst telegraph centralv oiiice, a second and a 'third telegraph central cnice, a telegraph channel connecting said rst oiilce to said second and third oices in parallel, a telegraph repeater in said channel, means at either said second or said third oirlce for transmitting telegraph communication signals over said channel to said rst oiiice, means in said second and said third ofiice for transmitting calling signals from said second oice to said'rst oice','and means in saidl second and third oice for transmitting recall signals from said second oflice to said rst office.

17. In a telegraph switching system, a iirstV telegraph central oince, a second and a third telegraph central oce, a telegraph channel connecting said iii-st oilice to said second and third offices in parallel, a` telegraph repeater in said channel, means at either said second or said third oice for transmitting telegraph communication signals over said channel to said rst ofce, means inV said second or said third oiice for transmitting channel calling signals from `said second or third cnice to said first ofiice, and means in said system for transmitting disconnect signals from said second or third office to said rst o'ice.

v18. Inra telegraph switching system, a rst telegraph central oiilce, a second and a third telegraph central cnice, a telegraph channel connecting said iirst oiiice to said secondV and third oices in parallel, a telegraph repeater in said channel, means at either said second or said third office for transmitting telegraph communication signals over said channel to said first ofiice, means in s-aid second oflice for transmitting recall signals from said second oiice to said first office, and means in said second office for transmitting disconnect signals from said second office to said first ofce.

19. In a telegraph switching system, a lirst telegraph central office, a second and a third telegraph central office, a telegraph channel connecting said nrst office to said second and third oflices in parallel, a telegraph repeater in said channel, means at either said second or said third oiflce for transmitting telegraph communication signals over said channel to s-aid first office, means in said system for transmitting channel calling signals from said second office to said first ofce, means in said system for transmitting recall signals from said second office to said rst office, and means in said system for transmitting disconnect signals from said second office to said first oice.

20. In a telegraph switching system, a first telegraph central switching oiiice, a second and a third telegraph central switching ofce, a telegraph channel connecting said first oiiice to said second and third ofiices in parallel, a telegraph trunk termination connected to said channel at each of said second and said third oiiices, means in each of said terminations for discriminating between a telegraph calling signal and a telegraph supervisory signal transmitted from said first oliice over said channel to said second and third o'ices, and means responsive to the answering of said calling signal at said second ofiice for effectively disconnecting said trunk termination from said third ofce.

21. In a telegraph switching system, a first telegraph central oiiice, a second and a third telegraph central oice, a telegraph channel connecting said first office to said second and said third oices in parallel, a telegraph repeater in said third offices for operating an indicator to said channel, means at each of said second and indicate that a call is incoming over said channel from said rst oflice, and means responsive to the answering of said call at said second office for restoring said indicator at said third oiilce to its original condition.

22. In a telegraph switching system, two separate telegraph switching trunk terminations, a telegraph calling signal control and a telegraph cord switching circuit supervisory signal control in each of said telegraph trunk terminations, said terminations in two separate central telegraph switching offices, said terminations connected in parallel to one end of a long distance telegraph trunk circuit, a telegraph repeater in said long distance telegraph trunk circuit, and means in said terminations, responsive to a signal trans'- mitted over said circuit, for registering a calling signal at each of said two terminations substantially simultaneously.

23. In a telegraph switching system, a telegraph channel including a repeater, a first telegraph trunk termination connected from a iirst end of said channel to a first telegraph central oice, a second telegraph trunk termination connected from said first end of said channel to a second telegraph central oflice, separate cord circuit supervisory signal controls in each of said terminations, means at either of said oces for answering a call incoming over said channel, and means at either of said offices responsive to said answering for disabling a telegraph communication path between the other of said offices and said channel.

24. In a telegraph switching system, a telegraph channel, a telegraph repeater in said channel, a first telegraph trunk termination connecting a first end of said channel to a first central telegraph switchboard, a, second telegraph trunk termination connecting said iirst end of said channel to a second central telegraph switchboard, separate cord circuit supervisory signal controls in each of said terminations, and means responsive to the answering of a call incoming through said termination at said first switchboard for impressing a busy condition on said trunk termination in said second switchboard.

25. In a telegraph switching system, a telegraph channel, a telegraph repeater in said channel, a iirst and a second telegraph trunk termination, at a first and a second manual central office telegraph switchboard, each of said switchbcards being at .a different oice, said terminations connected in parallel to each end of said channel,

separate cord circuit supervisory signal con' trols in each of said terminations, and means for transmitting a calling signal from either of said switchboards at one end of said channel to both of said switchboards at the other end of said channel.

26. A system in accordance with the preceding claim including uneans for responding to said signal at either switchboard in which it appears.

27. A system in accordance with claim 25 including means for effectively disconnecting the communication path to one of said switchboards in which said signal appears when the call is answered at the other of said switchboards in which said signal appears.

CARL A. DAHLBOM.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

